This invention relates to a process for the production of substantially closed-cell rigid foams containing urethane, urea and biuret groups, which show excellent adhesion to solid surfaces and to their use as insulating materials and/or for strengthening mechanical structural components.
Processes for the production of substantially closed-cell rigid foams containing urethane, urea and biuret groups are generally known. A review of known processes for the production of such rigid foams can be found, for example, in Kunststoff-Handbuch, Vol. VII, "Polyurethane", edited by R. Vieweg and A. Hochtlen, pages 504 et seq (Carl Hanser Verlag, Munchen 1966).
Substantially closed-cell rigid foams containing urethane, urea and biuret groups are generally produced by reaction of aromatic polyisocyanates with relatively high molecular weight polyols, preferably polyether or polyester polyols, in the presence of blowing agents, catalysts, flameproofing agents and other auxiliaries and additives. It is also known that, instead of polyether or polyester polyols, it is possible to use mixtures of these synthesis components. Such mixtures have been used because the production cost is reduced by mixing expensive polyester polyols suitable for the production of high-quality rigid polyurethane (PU) foams with inexpensive polyether polyols. Suitable low-viscosity polyester polyols for the production of rigid PU foams having favorable mechanical properties are described, for example, German Offenlegungsschrift 2,704,196.
It is also possible to use esters in the form of recycled products of relatively high molecular weight esters. In this case, the esters may also be used as relatively inexpensive components for reducing the cost of the mixtures. These inexpensive esters may be obtained, for example, by glycolysis of high molecular weight dimethyl terephthalate (DMT) polymers or by processing the distillation residues remaining behind in the production of DMT.
By using polyester polyols as the relatively high molecular weight polyhydroxyl compound in the production of rigid foams containing urethane groups, it is possible in particular to improve the flame resistance of the rigid foams and to reduce their brittleness.
In each of the processes mentioned above, fluorocarbon blowing agents are used to ensure that the desirable properties of rigid foams, including for example toughness, adhesion to surface layers and reduced flammability, are obtained.
As can be seen from the comparative Examples infra, rigid foams which have been produced without fluorocarbon blowing agents have significant disadvantages, including brittleness, inflammability and non-adhesion to surface layers.